In the case of the use of a bottom electrode with contact electrodes in the form of metal rods arranged on a base plate according to EP-B-0 058 817 and of the design of the hearth bottom in the form of a plurality of conductive or nonconductive layers consisting of refractory material built up one above another according to EP-B-0 156 126, the metal rods are used as anodes, while a central graphite electrode, arranged from the bottom, is connected as a cathode, which penetrates into the melting vessel through the pivotable cover.
For melting down scrap metal or other metallic charges, the arc first opens in the charge, and later in the melt covered by the slag.
The current now flows from the graphite electrode to the bottom electrode inserted in the center of the hearth bottom, and leads to continuous circulation and partial mixing of the molten steel and slag mixture located above the bottom electrode. It was observed that, especially in d.c. arc furnaces. both the slag and the melt located thereunder circulate from the outside to the inside in the direction of the arc, and that increased wear can be observed above the bottom electrode to the tamped, nonconductive hearth bottom due to the high specific current load of the cathode and anode.
In addition, it was determined that increased wear develops due to so-called pitting corrosion on the metal rods in the area of the upper, fire-side, nonconductive area consisting of refractory material.